Conventionally, an imaging apparatus such as a digital still camera is generally provided with an automatic exposure (AE) function for adjusting the amount of exposure automatically. Exposure control is performed by measuring the amount of light in the view, and, based on the measuring result, adjusting the aperture of the lens, the electronic shutter amount, and the gain of electrical signals output from the imaging element.
To measure the amount of light, a dedicated sensor or output signals of an imaging element is used. Examples of a method of measuring the amount of light includes the full-screen average metering method by which the average brightness in the whole screen is measured, the center-weighted average metering method by which the brightness at the center part in the screen is selectively measured, the multi-zone metering method by which the screen is divided and the average brightness in each area is measured, the spot metering method by which the brightness at an arbitrary position in the screen is measured, and the multi-spot metering method by which the brightness at a plurality of arbitrary positions in the screen is measured.
In spot metering, the part that is desired to be focused on or that needs to be focused on, i.e. the part of high importance, in the screen is detected, and the average brightness in the area is measured. Examples of parts of high importance include a face, a person, and a letter. Further, as for a vehicle-mounted imaging apparatus, examples of parts of high importance include a vehicle traveling on the road, a white line, and a road sign at the roadside.
Patent Literature 1 discloses a method of controlling exposure for detecting a white line. In the case where a vehicle is traveling on the road where there are sunlight and shade alternately, hunting occurs if the timing when sunlight and shade switch does not synchronize with the timing of exposure control. To prevent hunting, Patent Literature 1 discloses a technique of providing a limit value for an increase and a limit value for a decrease with respect to the amount of change in exposure control, and limiting the amount of change in exposure control to the limit values when the amount of change exceeds these limit values.
Further, Patent Literature 2 discloses a method of controlling the resolution and exposure in an area containing an object of high importance, for example, a vehicle traveling ahead, a person, and a road sign. An area with an object of high importance is determined based on how well the shape of the object in the image matches with the memorized shape of an object, and the camera is controlled so as to set the resolution and the exposure in the area are set at predetermined values. When there are a plurality of areas with an object of high importance, a weight is applied based on the type of an object of high importance (whether it is a person, a vehicle, or a road sign) and the position thereof in the screen (whether it is inside the road or outside the road, or whether it is near or far from the subject vehicle), and determines the exposure using the average result.